Aluminophosphate and metalloaluminophosphate molecular sieves, particularly silicoaluminophosphate (SAPO) molecular sieves, exhibit a wide range of framework types and are useful as catalysts in a variety of reactions. Of particular interest are the CHA framework type materials, such as SAPO-34, and the AEL framework type materials, such as SAPO-11 (see “Atlas of Zeolite Framework Types”, 2007, 6th Revised Edition). Thus CHA framework type molecular sieves have shown particular activity and selectivity in the conversion of oxygenates, such as methanol, to olefins, especially ethylene and propylene. Similarly, it is known that SAPO-11 molecular sieves catalyze hydroisomerization reactions of wax with high selectivity yielding lubricants with high viscosity index and low pore point. SAPO-11 has also been found to be useful as a catalyst in naphtha cracking where it is found to give a high selectivity for propylene. Intergrowths of different framework type molecular sieves have also shown considerable promise as catalytic materials, for example intergrowths of CHA and AEI framework type materials, such as EMM-2, have been found to be highly attractive catalysts for the conversion of oxygenates to olefins (see, for example, U.S. Pat. No. 6,812,372).
The synthesis of aluminophosphate and metalloaluminophosphate molecular sieves is normally conducted by initially producing a synthesis mixture comprising water, an organic template, typically a nitrogen containing organic base, such as a quaternary ammonium salt or hydroxide, an aluminum oxide, phosphoric acid and optionally a source of silicon or other metal. The resulting gel mixture is then subjected to hydrothermal conditions in a sealed vessel to induce crystallization. The crystalline product is then recovered by filtration or centrifugation.
The organic template, which is sometimes referred to as a structure directing agent because of its role in determining the framework type of the molecular sieve product, also plays the role of moderating the pH of the synthesis gel mixture. However, the organic template, particularly where the template is a quaternary ammonium compound, is frequently the most costly ingredient used in the synthesis mixture. There is therefore significant interest in finding improved and less expensive templating systems for the production of aluminophosphate and metalloaluminophosphate molecular sieves. For example, in the case of CHA framework type silicoaluminophosphates, triethylamine, tetraethylammonium hydroxide (TEAOH) and morpholine have all been found to be suitable templating agents.
More recently, in U.S. Patent Application Publication Nos. 2003/0231999, 2003/0232006, and 2003/0232718, we have shown that CHA framework type aluminophosphates and silicoaluminophosphates can be synthesized in the presence of at least one template containing one or more N,N-dimethylamino moieties having the structure (CH3)2N—R wherein R linear or branched alkyl group, a linear or branched alcohol, or a linear or branched amine. Suitable templates include, but are not limited to N,N-dimethylethanolamine, N,N-dimethylbutanolamine, N,N-dimethylheptanolamine, N,N-dimethylhexanol-amine, N,N-dimethylethylenediamine, N,N-dimethylpropyelenediamine, N,N-dimethylbutylenediamine, N,N-dimethylheptylenediamine, N,N-dimethyl-hexylenediamine, dimethylethylamine, dimethylpropylamine, dimethylheptyl-amine and dimethylhexylamine.
In addition, U.S. Patent Application Publication No. 2004/0253163 discloses the synthesis of silicoaluminophosphate molecular sieves having the CHA framework type employing a directing agent having the formula:R1R2N—R3 wherein R1 and R2 are independently selected from the group consisting of alkyl groups having from 1 to 3 carbon atoms and hydroxyalkyl groups having from 1 to 3 carbon atoms and R3 is selected from the group consisting of 4- to 8-membered cycloalkyl groups, optionally substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms; and 4- to 8-membered heterocyclic groups having from 1 to 3 heteroatoms, said heterocyclic groups being optionally substituted by 1 to 3 alkyl groups having from 1 to 3 carbon atoms and the heteroatoms in said heterocyclic groups being selected from the group consisting of O, N, and S. Preferably, the directing agent is selected from N,N-dimethyl-cyclohexylamine, N,N-dimethyl-methylcyclohexylamine, N,N-dimethyl-cyclopentylamine, N,N-dimethyl-methyl-cyclopentylamine, N,N-dimethyl-cycloheptylamine, N,N-dimethyl-methylcyclo-heptylamine, and most preferably is N,N-dimethyl-cyclohexylamine.
U.S. Pat. No. 3,766,093 discloses synthesis of the aluminosilicate zeolite ZSM-5 in the presence of tetrapropylammonium hydroxide (Example 1) and in the presence of a mixture of tri-n-propylamine and n-propyl bromide (Examples 3 and 4).
However, while trialkylamines are generally less expensive than their quaternary ammonium counterparts, the latter exhibit some advantages over amines in the areas of improved control of acid site density, acid site distribution and crystal size of the resultant molecular sieve. According to the present invention it has now been found that the advantages of quaternary ammonium compounds in molecular sieve synthesis can be achieved without their concomitant cost disadvantage by producing the quaternary ammonium compounds in situ in the synthesis mixture by reaction between a tertiary amine and an alkylating agent, especially an alkyl phosphate. When an alkyl phosphate is employed as the alkylating agent for, the alkyl phosphate not only converts the trialkylamine into a quaternary ammonium compound but also provides a source of phosphate for the synthesis. In this way, the amount of phosphoric acid added to the synthesis mixture can be reduced, so that the initial pH of the gel tends to be higher, which favors the dispersion of the silicon in the resulting crystalline product.